The subject invention generally concerns the field of data buses in test and measurement instruments and specifically concerns data bus expansion between two test and measurement instrument chassis.
Modern test and measurement instruments have the capability of monitoring a plurality of channels simultaneously. For example, when a TLA 720 Logic Analyzer, manufactured by Tektronix, Inc., Beaverton, Oreg., is equipped to capacity with five 136-channel modules, it is capable of monitoring up to 680 channels. When it is desirable to monitor even more channels, one must employ an expansion chassis to house additional data acquisition cards.
In order to utilize such an expansion chassis, it is necessary to couple address lines and data lines between a primary (or master) unit and an expansion (or slave) unit. Data flow on the data lines is from the master unit to the expansion unit during data-write operations, and from the expansion unit to the master unit during data-read operations. That is, one must provide an address driver and data receiver/driver interface board at the primary unit and an address receiver and data receiver/driver interface board at the expansion unit, and then one must connect the two interface boards with one or more cables containing hundreds of wires.
This arrangement gives rise to several problems. First, from the user""s point of view, one must take care that the two interface boards are not mistakenly inserted into the wrong chassis. Placing a primary side address driver interface board into an expansion unit will, at the very least, cause improper operation of the system, if not damage to the system. Second, some systems (notably VXI bus) require a controller card to occupy slot 0, and driven cards to occupy slots other than 0. Different circuitry must be provided on slot 0 cards (with respect to non-slot 0 cards) to properly interface with the VXI bus system. Thus, from the manufacturers point of view, it is necessary to design, build, stock, and ship two different interface boards.
What is needed is a symmetric and reversible cable and circuit board arrangement that detects its role as a primary-side card or a secondary-side card, and operates correctly in either orientation.
Apparatus for data bus expansion between two instrument chassis comprises a first interface circuit, a connecting cable, and a second interface circuit. Detection circuitry in each interface circuit detects slot position. Slot position determines direction of address and data flow for each card. One of said first and second interface circuits is connected to a controller slot in a first chassis, and the other of said first and second interface circuits is connected to a non-controller slot in a second chassis. Bus interface circuitry for signal and identification are reconfigured in accordance with slot determination.